As dwellings, commercial buildings, and other structures become less permeable to environmental air, air pressure differentials can arise. Some of these structures can include air flow systems, including ventilation systems, so that a portion of the air within the structure can be exhausted to the outside environment. In some structures, at least partially depending on the inclusion of a make-up air system and the rate at which air exits the structure, negative pressure can be generated within the structure. Negatively pressurized structures can experience exhaust gas inflow and some increases in potentially harmful compounds.